Degassing of metals or alloys



United States Patent 3,321,306 DEGASSING F METALS OR ALLOYS Howard KnoxWerner, New Lambton, New South Wales, Australia, assigner, by mesneassignments, to Conzinc Riotinto of Australia Limited, Melbourne,Victoria, Australia, a corporation of Victoria Filed Aug. 7, 1964, Ser.No. 388,203 Claims priority, application Australia, Aug. 13, 1963,34,188/ 63 Claims. (Cl. 75-49) This invention relates to improvements inthe degassing of molten metals and alloys, and is concerned particularlywith a method whereby the degassing is effected continuously.

Several methods are known for removing occluded or otherwiseincorporated gases in molten metals or alloys. Practically all of thesemethods, even those designated continuous, are designed for degassingsteel within or as delivered from individual ladles. They are therefore,in reality, semi-continuous or intermittent in Operation.

This present invention has been developed primarily for the continuousdegassing of metals or alloys which are either produced continuously -orare delivered continuously from a holding furnace or from a continuousmelting furnace. The method and apparatus of the invention can, however,be used in a semi-continuous manner for degassing a metal or alloy whichis being transferred continuously from a ladle or other holding vesselto another holding vessel or furnace or to the tundish of a continuouscasting machine. rIhe invention is particularly well suited to thedegassifcation of molten steel being delivered continuously from aholding furnace to the tundish of a continuous casting unit.

The principles utilised in this invention are developed from thoseinvolved in a syphon comprising an inverted U-tube or the like having`an inlet leg connected to a first vessel containing molten metal oralloy to be degassed, an outlet leg delivering degassed metal or alloyto a second vessel, and a source of vacnum or reduced pressure connectedto the U-tube.

The molten metal or alloy to be degassed is caused to fiow continouslyunder the action of a vacuum or partial vacuum, from the first vessel,which may -be part of a holding furnace or a launder from a continuousproduction furnace, up over the hump of the inverted U syphon, to thesecond vessel, which may be part of the tundish of a continuous castingmachine or a launder or a holding furnace.

As suction is applied, atmospheric pressure on the liquid in the firstvessel forces it up into the inlet leg until the metal height thereinplus the pressure in the unit balances the atmospheric pressure. Theheight of the hump is appropriately arranged to be slightly lower thanthe height to which the metal is lifted, so that the metal ows -acrossthe hump and down the outlet leg of the inverted U-tube. With the vacuumVheld more or less constant and the level of metal in the inlet legmaintained steady, the inverted U-tube then functions like a syphon, andthe flow of metal in the syphon continues so long as and at the samerate as the metal is withdrawn from the second vessel into the castingmachine or other device.

We have discovered that the rate of evolution of dissolved and ocoludedgases from the molten metal or alloy in the system above described bearsa fairly direct relationship to the amount of surface area of liquidmetal exposed to the vacuum or reduced pressure, yand the presentinvention is concerned, inter alia, with methods of and means forsubstantially increasing both the 'icc amount of liquid surface exposedto the vacuum and the time of exposure.

In one general form the invention is a method of degassing metals oralloys which comprises elevating the molten metal or alloy under reducedpressure, up a barometric inlet leg into a degassing chamber, subjectingthe molten metal or alloy to turbulence and cascading :action to thedegassing chamber by flowing it under gravity down an extended generallysloping surface having one or more recesses in which a pool or pools ofmolten metal or alloy are contained, the molten metal or alloy beingcaused to fiow through the said pool or pools, and causing the degassedmolten metal or alloy to flow out of the degassing chamber through theIbarometric outlet leg.

The molten metal or alloy is subjected to turbulence or cascading actionduring its passage through the degassing zone. The molten metal or alloyin flowing over the extended surface in the degassing zone is subjectedto reduced pressure over an extended period, and maximum removal ofdissolved or occluded gases from the metal or alloy is thereby effected.After its passage through the degassing zone the lmolten metal or alloyis preferably caused to cascade in the form of a thin film or indroplets within the outlet leg of a syphon.

In a further form the invention is apparatus for degassing metals oralloys which comprises a first vessel containing molten metal or alloyto be degassed, an inlet leg of ya syphon unit dipping into the saidmolten metal or alloy, a second vessel containing degassed molten metalor alloy, an outlet leg of the lsyphon unit dipping into the degassedmolten metal or alloy, an intermediate member disposed between andcommunicating with the inlet leg and outlet leg, a degassing zone in theintermediate member, means for connecting the degassing zone to a sourceof reduced pressure, an extended generally sloping surface in theintermediate member over which the molten metal or alloy is fiowed undergravity, and one or more recesses in the extended generally slopingsurface in which a pool or pools of molten metal or alloys arecontained.

The syphon unit preferably comprises an inverted U-tube with the openlower ends of its inlet leg -and outlet leg adapted to dip respectivelyinto the untreated molten metal or alloy in the first vessel (which maybe a ladle, feed launder, holding furnace, or other vessel) and into thedegassed molten metal or alloy in the second vessel (which may -bea'casting machine tundish, a launder, a holding vessel or the like). Theoutlet leg is preferably longer than the inlet leg, for reasons whichwill be apparent from the ensuing description. The space inside theU-tube is connected to :a source of vacuum or reduced pressure. The flowof molten metal or alloy through the syphon unit may -be continuous orintermittent.

The degassing zone of the unit may comprise an extended surface-whichslopes generally from the inlet leg and which is provided with one ormore recesses containing pools of molten metal or alloythrough which themolten metal or alloy iiows or cascades, the said metal or alloy beingthereby subjected to turbulence so that a large surface area of saidmetal or alloy is exposed to reduced pressure yduring the passage of themetal or alloy over said extended surface, and the residence time of themetal or alloy in the degassing zone is thus relatively long. The lowerinternal surface of the hump portion of the U-tube comprising thedegassing zone may be formed with transverse ridges, which both slowdown the rate of flow of molten metal or alloy from the degassing zoneor cascading action in the flowing liquid.

Means are also preferably provided to effect an overflow of molten metalor alloy from the degassing zone into the outow leg, and this means maycomprise an overflow lip or Weir at the exit end of the said extendedsurface over which the molten metal or alloy is caused to cascade in theform of a thin film or in droplets as it falls to the level of thedegassed molten metal or alloy in the outlet leg.

In another form the invention consists of apparatus as above describedtogether with means for admitting into one or both of the inlet andoutlet legs of the inverted U-tube a gas which has negligible solubilityin and no deleterious effect on the molten metal or alloy.

The term tube as used in this specification is intended to include notonly tubes hav-ing circular or oval cross sections, but also thosehaving other cross sections, for example square or rectangular.Furthermore the tube may have `appropriate vacuum-tight joints at ornear the junction ofthe inlet and outlet legs, respectively with thehump section of the inverted U, or at the other desired positions.

Different forms of the invention are shown in the accompanying drawings,wherein:

FIGURE 1 is a view in sectional elevation of one form of apparatus forthe continuous degassing of molten metals or alloys,

FIGURE 2. is a view in sectional elevation of another form of suchapparatus, and

FIGURE 3 is a view in sectional elevation of a further form of suchapparatus.

In all of the illustrations the same reference numerals are used toindicate like or corresponding parts.

Referring to FIGURE 1, the reference numeral 5 indicates a vessel, suchas a launder or holding furnace, containing molten metal or alloy 6which is to be degassed. The vessel 7, which may be the tundish of acontinuous casting machine, contains degassed metal or alloy 8.

The degassing unit comprises an inverted tubular syphon structure 9consisting of an inlet leg 10 the lower end of which dips into themolten metal 6 in the vessel 5', a degassing zone 11 for-med within anintermediate member 12 which is connected to the upper end of the inletleg 10, and an outlet leg 13, of greater length than the inlet leg 10,which is connected at its upper end to the lower or outlet end of thedegassing zone 11, the lower end of the outlet leg 13 being arranged todip into the degassed molten metal 8 in the vessel 7.

The syphon structure 9 may be constructed of metal and lined withrefractory material.

The upper wall of the intermediate member 12 is provided with anaperture to which is connected a pipe 14 which is connected to a sourceof vacuum or reduced pressure.

The lower sloping surface 22' of the intermedi-ate member 12 is formedwith a series of small transverse ridges 21, and a lip or overflow 16 isprovided at the lower outlet end of the said intermediate member 12.

Gas inlet tubes 22 and 23 are provided by which an inert gas may beadmitted into the inlet leg and outflow leg 13 respectively.

A baflie or deflecting plate 24 is provided in the degassing zone 11which serves to prevent the in-rushing metal from splashing or sprayingtoo far into the evacuated degassing zone 11 and possibly being suckedinto the pipe 14 leading to the vacuum pumping system. If necessary, afurther bafe plate 25 can be arranged adjacent to the entry to pipe 14.

A sight tube 26 is tted in the wall of the syphon unit 9, by means ofwhich the splashing and cascading action of the molten metal can beobserved.

In operation, a vacuum is applied through the pipe 14 to the interior ofthe syphon structure 9 and molten metal or alloy 17 to be degassed iselevated in the inlet leg 10 to a height h. The molten metal ows overfrom the upper end of the inlet leg into the degassing zone 11 and themetal 18 is then caused to cascade in succession over the ridges 21under gravity until it reaches the lip or overflow 16, and the saidlmetal then flows over said lip 16 and falls in the form of a lm ordroplets 19 or both until it enters the body 20` of molten metal in theoutlet leg 13.

During the ow of the molten metal over the ridges 21 `and through thepools of metal formed in the recesses between the ridges 21 and duringits free fall in the outlet leg 13, and said metal is subjected toturbulence and/or cascading action which ensures that a large surfacearea of the molten metal is exposed to the reduced pressure, and thatthe molten metal is exposed to such reduced pressure for a substantialperiod, that is, there is a relatively long residence time of the metalin the degassing zone 11. By this means it has been found possible tosubstantially improve the efiiciency of the degassing operation.

The amount of free fall of the molten metal in the outlet -leg 13depends on the length or height of the said outlet leg relative to thatof the inlet leg 10. In the degassing of Imolten steel the inow leg 10will usually be between 5 and 5.5 feet long, thus allowing a barometricleg height h of 56 to 57 inches when the vacuum in the system isequivalent to between 0.9. and 0.5 torr. The outflow leg 13 canconveniently be between 7 and l0 feet long. Allowing for the outlet leg13 being immersed 0.5 foot to .1 foot in the molten steel in vessel 7and a barometric height h of 4.75 feet, the fall of films or droplets ofmolten metal in the outlet leg 13 is approximately between 1.5 and 4feet.

In this specification the term inert gas shall be understod to includenot only gases like argon and helium, which are classied as inert inclassical chemistry, but any other gas which has negligible solubilityin the metal or alloy to be degassed and which, furthermore, will notreact detrimentally with it.

When inert lgas is admitted into the inlet leg 10, it has the effect ofreducing the apparent density of the metal 17 in this leg and so causesit to rise to a higher level and at greater velocity than wouldotherwise be the case. Under such circumstances, the metal may spray outof the opening from the inlet leg 10 into the zone 11 in the form oftine droplets. This also assists the degassing, which is also assistedby the washing action that a second gas may exert in relation to the gasor gases which it is de-v sired to remove from the liquid metal oralloy.

An inert gas admitted to the outlet leg 13 as for example at 23, acts asa type of scavenger, washing out and carrying upwards into the zone 11any small amounts of residual occluded gas or gases which may haveresisted removal by the action of cascading under vacuum in zone 11.

The inert gas may be admitted through the walls of the refractory legs10 and 13 by means of fluid cooled apertures (not shown). Alternatively,the inert gas may be admitted through a separate refractory pipe asshown at 23 in FIGURE l.

Another form of the invention is shown in FIGURE 2. In this form, one ormore pools 27 of molten metal is -or are formed in the degassing zone 11by appropriate depress-ions or recesses 28 in the sloping internal lowersurface of the member 12. If these pools 27 have an appreciable totalvolume, they effectively increase the residence time of the liquid metal18 in the degassing zone 11 and so permit greater time for gas bubblenucleation and gas bubble removal. The surfaces of the refractory`depressions 28 are preferably either porous or roughened, :since it isfound that micro bubbles are more read-ily nucleated at micropores inthe refractory surface than they are within the liquid metal itself orat a smooth solid surface.

As an aid to degassing of low carbon steel, finely particulate carbon 29may be fed through an appropriately sealed tubular -powder chargingdevice 30 into the molten stream of metal as it leaves the inlet leg 10and flows into the zone 11. The carbon particles 29 become stirred intothe bath of molten metal and in addition to providing micro-porous solidsurfaces on which gas bubbles can nucleate, the carbon reacts with anyoxygen ions in the Ibath forming carbon monoxide. The evolution of this-g-as helps to sweep hydrogen and nitrogen out of the molten steel.

In the form of the invention shown in FIGURE 3 a series of pools orArecesses 27 are formed in stepewise formation by means of refractoryweirs or walls 31. Apertures 32 are formed in the walls 31 through whichthe molten metal is caused to cascade as shown at 33 from each pool orrecess 27 into the next succeeding lower pool or recess 27. The moltenmetal then cascades or overflows over the lip v16 into the outlet leg 13as described above.

The dimensions of the inverted U-shaped degassing structure aredetermined by the quantity lof liquid metal to be degassed in any unitof time and also by the lift produced by the vacuum applied. Naturally,with any given degree of vacuum, the lift will be greater with a lowerspecific gravity metal. Thus, in the case of aluminium, and Withoutinert gas admission, the lift would need to be over 21/2 times thatrequired for molten steel. For this reason, the application of thisinvention to degassing of aluminium requires ample insulation around thelegs and 13 and the intermediate member 12, otherwise lheat losses maybe too great. The use of chlorine gas injection into the inlet leg 10,and possibly also into the outlet leg 13, makes possible a considerablereduction -in the height necessary for adequate degassing of aluminium.

'I'Io counteract heat losses it is possi-ble, and may in some cases bedesirable, to apply heat electrically to the molten metal as it flows upthe inlet leg 10 and/or as it passes through the zone 11. This canconveniently be done by means of induction coils or by resistanceelements surrounding or appropriately embedded in the refractorymaterial of the degassing unit.

The refractory material used should he a dense variety and appropriateto the metal or alloy being degassed. To assist in forming an adequateseal, the refractory is conveniently encased in steel or otherappropriate metal, except at the bottom ends of the legs 10 and 13 wherethey dip into the molten untreated metal 6 and the degassed metal 8,respectively. It is appropriate to iirst dip these lower ends of theinlet and outlet =legs 10 and 13 into a ladle or vessel (not shown)containing an appropriate slag or glass. This coating helps both toprotect and to seal the refractory at these positions.

The inverted U-shaped degassing unit may be supported along the lengthof the intermediate member 12 by a steel cage (not shown).Alternatively, rings (not shown) may be welded to the steel casing atappropriate positions so that hooks and chains (not shown) can beattached for the purpose of lowering the degassing equipment intoposition and adjusting its height and the angle of inclination of theintermediate member 12 during operation.

When degassing -is complete or some repair operation necessitatesterminating the flow, a valve (not shown) in the vacuum line 14 can beslowly opened, and air gradually admitted to the degassing unit,allowing the metal in the inlet and outlet legs 10 and 13 to iiow backinto the vessels 5 and 7 respectively. In the form of the inventionincorporating pools 27 in the degassing zone 11, (see FIGURE 2) it maybe necessary to tilt the degassing unit to discharge the metal remainingin the pools 27 before the apparatus is lifted out of position.

In starting up the ydegassing unit it is of course necessary to have aquantity of the liquid metal or alloy 8 in the launder, tundish or othervessel 7 .so that an effective seal is achieved in the outow leg 13.

It is possible, and in some applications advantageous, to maintain aprotective layer of slag on the molten metal 6 or alloy 6 in the vessel5. If the level ofthe entry tothe inlet leg 10 is Well down in themolten metal 6, no slag will be sucked in with the metal and thusslag-free and degassed metal 8 will be discharged into the vessel 7.

The advantages of the invention are:

(i) Its relative simplicity,

(ii) Its applicability vto the degassing of molten metal or alloy beingproduced continuously,

(iii) Its suitability for interposition between a metalproducing furnaceand a continuous casting machine,

(iv) Its ease of removal and replacement.

In connection with (iii), it may in some cases be advantageous to havetwo or more degassing units operating in parallel, these being fed fromthe one production furnace and discharging the degassed product into theone continuous casting machine. Should one degassing unit need to bereplaced, there is a continuing delivery of degassed metal or alloy viathe other unit(s).

The following example illustrates the invention:

Low carbon (0.12% C) semi-killed steel melted under Oxy-oil burners anddeliverately contaminated with hydrogen and nitrogen was degassed in adegassing unit of the type shown in FIGURE 1 with the degassing zone 11being 3 feet long and with a vacuum of 0.5 torr and a free fall in theoutow leg of approximately 3 feet.

Before degassin-g, the steel contained 0.011% nitrogen and 5 parts permillion of hydrogen. After degassing, the steel contained 0.003%nitrogen and 2.5 parts per million of hydrogen. With larger equipment,high vacuum and by use of inert gas injection, the nitrogen and hydrogencontents can be reduced to substantially lower values.

I claim:

1. A method for degassing molten metallic material which compriseselevating the molten metallic material under reduced pressure up abarometric inlet leg into a reduced pressure degassing chamber,subjecting the molten metallic material to turbulence and cascadingaction for exposing a large surface area to the reduced pressure in thedegassing chamber by ilowing it under gravity in a relatively thin iilmover at least one ridge in the surface of the bottom of the degassingchamber, holding the thus exposed molten material for a relatively longtime in said degassing chamber by flowing it into at least one recesstherein which has an appreciable total volume to form a pool of moltenmetallic material for permitting greater time for gas bubble nucleationand gas bubble removal, the molten metallic material being caused to iowthrough the said pool, and causing the degassed molten metallic materialto flow out of the recess over a further ridge and out of the degassingchamber through the barometric outlet leg.

2. A method according to claim 1 wherein the metallic material is causedto overow from the degassing zone into the barometric outlet leg and tofall freely in the form of droplets or film into the molten metallicmaterial in said barometric outlet leg.

3. Apparatus for degassing molten metallic material which comprises alirst vessel containing molten metallic material to be degassed, asyphon unit having an inlet leg dipping into the said molten metallicmaterial, a second vessel containing degassed molten metallic material,the syphon unit having an outlet leg dipping into the degassed moltenmetallic material in the second vessel, said syphon unit having anintermediate member disposed between and communicating with the inletleg and outlet leg, a degassing zone in the intermediate member, meanson said intermediate member for connecting the degassing zone to asource of reduced pressure, an extended generally sloping surface in theintermediate member over which the molten metal or alloy is owed undergravity, said intermediate member having at least two ridges thereon andat least one recess of appreciable total volume in the extendedgenerally sloping surface between said ridges in which a pool of moltenmetallic material is contained.

4. Apparatus according to claim 3 and having means in said intermediatemember for causing the molten metallic material to cascade into and outof each pool.

5. Apparatus according to claim 3 and having a plurality of ridges onthe extended surface between which the recess is formed.

6. Apparatus according to claim 3 wherein the outlet leg of the syphonunit is longer than the inlet leg.

7. Apparatus according to claim 3 and having means in said intermediatemember for causing the molten metallic material to cascade in the formof droplets or ilm within the outlet leg onto the molten metal or alloyin said outlet leg.

8. Apparatus according to claim 3 and having an overflow weir at thelower end of the extended generally sloping surface over which themolten metallic material cascades into the outlet leg.

9. Apparatus according to claim 3 and having means connected to saidinlet leg and said outlet leg for admitting inert gas to the metal oralloy in the inlet leg and outlet leg.

10. Apparatus according to claim 3 and having means in said intermediatemember for admitting particulate material for the degassing zone.

References Cited by the Examiner UNITED STATES PATENTS 1,554,368 9/1925Rackoi et al. 266-34 X 1,921,060 8/ 1933 Williams 75-49 2,054,922 9/1936 Betterton et al. 266-34 2,587,793 3/1952 Waldron 266-34 X 3,022,0592/ 1962 Aarders 75-49 3,202,409 8/ 1965 Farrer 266-34 3,230,074 1/1966Roy et al. 266-34 X HYLAND BIZOT, Primary Examiner.

DAVID L. RECK, Examiner.

H. W. TARRING, Assistant Examiner.

1. A METHOD FOR DEGASSING MOLTEN METALLIC MATERIAL WHICH COMPRISES ELEVATING THE MOLTEN METALLIC MATERIAL UNDER REDUCED PRESSURE UP A BAROMETRIC INLET LEG INTO A REDUCED PRESSURE DEGASSING CHAMBER, SUBJECTING THE MOLTEN METALLIC MATERIAL TO TURBULENCE AND CASCADING ACTION FOR EXPOSING A LARGE SURFACE AREA TO THE REDUCED PRESSURE IN THE DEGASSING CHAMBER BY FLOWING IT UNDER GRAVITY IN A RELATIVELY THIN FILM OVER AT LEAST ONE RIDGE IN THE SURFACE OF THE BOTTOM OF THE DEGASSING CHAMBER, HOLDING THE THUS EXPOSED MOLTEN MATERIAL FOR A RELATIVELY LONG TIME IN SAID DEGASSING CHAMBER BY FLOWING IT INTO AT LEAST ONE RECESS THEREIN WHICH HAS AN APPRECIABLE TOTAL VOLUME TO FORM A POOL OF MOLTEN METALLIC MATERIAL FOR PERMITTING GREATER TIME FOR GAS BUBBLE NUCLEATION AND GAS BUBBLE REMOVAL, THE MOLTEN METALLIC MATERIAL BEING CAUSED TO FLOW THROUGH THE SAID POOL, AND CAUSING THE DEGASSED MOLTEN METALLIC MATERIAL TO FLOW OUT OF THE RECESS OVER A FURTHER RIDGE AND OUT OF THE DEGASSING CHAMBER THROUGH THE BAROMETRIC OUTLET LEG.
 3. APPARATUS FOR DEGASSING MOLTEN METALLIC MATERIAL WHICH COMPRISES A FIRST VESSEL CONTAINING MOLTEN METALLIC MATERIAL TO BE DEGASSED, A SYPHON UNIT HAVING AN INLET LEG DIPPING INTO THE SAID MOLTEN METALLIC MATERIAL, A SECOND VESSEL CONTAINING DEGASSED MOLTEN METALLIC MATERIAL, THE SYPHON UNIT HAVING AN OUTLET LEG DIPPING INTO THE DEGASSED MOLTEN METALLIC MATERIAL IN THE SECOND VESSEL, SAID SYPHON UNIT HAVING AN INTERMEDIATE MEMBER DISPOSED BETWEEN AND COMMUNICATING WITH THE INLET LEG AND OUTLET LEG, A DEGASSING ZONE IN THE INTERMEDIATE MEMBER, MEANS ON SAID INTERMEDIATE MEMBER FOR CONNECTING THE DEGASSING ZONE TO A SOURCE OF REDUCED PRESSURE, AN EXTENDED GENERALLY SLOPING SURFACE IN THE INTERMEDIATE MEMBER OVER WHICH THE MOLTEN METAL OR ALLOY IS FLOWED UNDER GRAVITY, SAID INTERMEDIATE MEMBER HAVING AT LEAST TWO RIDGES THEREON AND AT LEASST ONE RECESS OF APPRECIABLE TOTAL VOLUME IN THE EXTENDED GENERALLY SLOPING SURFACE BETWEEN SAID RIDGES IN WHICH A POOL OF MOLTEN METALLIC MATERIAL IS CONTAINED. 